94 Percent Drop in Symptomatic COVID Cases Seen Among Vaccinated, Biggest Israeli Study Shows

The Pfizer COVID vaccine, the study found, was equally effective for all age groups, including people aged 70 and older

Ido Efrati
Ido Efrati
A young man receives the Pfizer coronavirus vaccine in Bnei Brak on Friday.
A young man receives the Pfizer coronavirus vaccine in Bnei Brak on Friday.Credit: Tomer Appelbaum
Ido Efrati
Ido Efrati

The Pfizer coronavirus vaccine has produced a 94 percent drop in symptomatic coronavirus cases, a study published Sunday by the Clalit health maintenance organization's research institute says.

The study analyzed 1.2 million Clalit members, including 600,000 who had received the Pfizer vaccine and 600,000 who had not. The inoculated group produced 94 percent fewer symptomatic COVID-19 cases, and 92 percent fewer cases of serious illness.

The vaccine, the study found, was equally effective for all age groups, including people aged 70 and older – a group for which Pfizer’s clinical trials had not produced conclusive data, as too few people of this age participated.

The vaccinated group included 170,000 people aged 60 and older and 430,000 people aged 16 to 59. The unvaccinated group was carefully chosen so that each person in the vaccinated group was matched with a similar person in the control group based on a long list of criteria, including risk of infection, risk of becoming seriously ill, general health and place of residence.

If members of the control group got vaccinated during the course of the study, they were switched into the vaccinated group and new people were added to the control group. The data was also controlled for the influence of factors other than the vaccine, including the coronavirus lockdown.

While the study reinforced that the vaccine is highly effective in preventing symptomatic infection a week or more after the second dose, it also showed that it was highly effective in preventing serious illness, which clinical trials were unable to prove, since there were too few cases of serious illness among the participants for the data to be statistically significant.

The study found that the vaccine is between 91 and 99 percent effectiveness against serious illness seven days after the second dose. But the researchers said they expected the effectiveness to rise after 14 days.

Prof. Ran Balicer, the director of the Clalit Research Institute, said that the researchers had developed various tools and methodologies to control for various factors that can influence studies based on real-world data.

“We had to cope with challenges related to the clear differences between the vaccinated and unvaccinated populations and the changes in them over time, as well as the influence of the lockdown, which was imposed and then lifted during this period,” he said.

“Together with our colleagues at Harvard University, we conducted a series of tests to validate the results, and it’s now unequivocally clear that Pfizer’s coronavirus vaccine is extremely effective in real life a week after the second dose, just as was found in the clinical trials."

Even though the study is still ongoing – additional results are due to be released in the coming weeks – the publication of these preliminary results was intended to show the unvaccinated population that the vaccine is in fact extremely effective. Israel is currently a world leader in terms of its per capita vaccination rate, but most medical experts agree that vaccinating the rest of population is essential to overcome the coronavirus.

Clalit, the country’s largest HMO, has already administered over 3.2 million doses of the vaccine. That number includes second doses, however, so the number of people it has vaccinated is lower.

The researchers who participated in the Clalit study are Dr. Noa Dagan, Dr. Noam Barda, Dr. Eldad Kepten, Oren Miron, Shay Perchik, Prof. Mark Katz and Balicer. The researchers from Harvard were Prof. Miguel Hernan, Prof. Marc Lipsitch and Prof. Benjamin Rice.

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